1. Field of the Invention
The present invention relates to scintillation counting and, more particularly, to apparatus for positioning a standard radioactive source proximate the counting chamber of a liquid scintillation counter.
2. Description of the Prior Art
Liquid scintillation techniques have been widely adopted to measure the count rate or activity of samples containing radionuclides. It is well known that materials present in the sample can affect the process by which the liquid scintillation solution responds to the radionuclide radiation thereby causing a reduction in the measured count rate. This is commonly referred to as quenching, and numerous techniques have been developed in the prior art to measure and correct for quench in order to accurately determine the activity of the sample. One of the more widely adopted techniques is that of external standard channels ratio ESCR) determination exemplified by U.S. Pat. No. 3,381,130. A second and pioneering technique, commonly termed H-number determination, represents an improvement over the ESCR approach and is exemplified by U.S. Pat. No. 4,075,480. In both of the foregoing techniques, the sample is counted twice, once by itself and once while being irradiated by a known or standard radioactive source.
Typically, in each of the above techniques, a standard radioactive source is disposed in a carrier tube or conduit and is shifted back and fourth within the conduit between a shielded or rest location remote from the counting chamber and a counting or operating location adjacent the counting chamber. The radioactive source is usually a source of gamma radiation, such as cesium-137, incorporated within a suitable container such as a metal ball or pellet which will slide freely within the conduit. When a sample positioned is in the counting chamber, the radioactive source is shifted to its operating location to irradiate the sample, and the resulting scintillations (light flashes) emanating from the sample are counted. Thereafter, the source is returned to its shielded location and the scintillations produced solely by the radioactive sample are counted. This two-step counting procedure is repeated for each sample to be analyzed.
While various arrangements for shifting the radioactive source within the conduit have been adopted in commercial liquid scintillation counters, they have not proven satisfactory in all respects. In one approach, a pneumatic pump and a plurality of solenoid controlled valves are connected to the conduit. In a first operating mode, the pump propels the radioactive source from its shielded location to its operating location. Then, at the conclusion of the first counting step, the valves are selectively actuated to reverse the direction of air flow in the conduit such that the pump propels the source back to its shielded location. The second counting step is then conducted for the sample alone. The use of solenoid-controlled valves to control the direction of air flow results in a relatively expensive and mechanically complex control system which, in the course of switching operation, introduces undesired noise signals into the scintillation counting system.
A second and simpler approach utilizes only one-way air flow in the conduit to propel the radioactive source to the operating position at the counting chamber. After the first step of counting the irradiated sample, the air flow is terminated, and the radioactive source falls by gravity to the shielded location. The second counting step is then conducted for the sample alone. Unfortunately, it has been found with this arrangement that the radioactive source often sticks in the operating position adjacent the counting chamber and sample. When this happens, the source continues to irradiate the sample during the second counting step. Since the scintillations produced by the source totally swamp scintillations produced by the sample alone, under such circumstances the second counting step produces totally erroneous counting information.
In addition to the foregoing, the pneumatic systems may allow the radioactive source to oscillate or vibrate at the operating position adjacent the sample and may fail to identically position the source adjacent each successive sample elevated into the counting chamber. An oscillating or otherwise inaccurately positioned source can introduce errors in the sample counting and calibration procedures.
Finally, in the pneumatic positioning systems, the radioactive source is subject to shock and vibration during shifting back and forth and particularly when striking stops at the end of the conduit. This increases the likelihood that radioactive material will escape from the source container.
In another approach for shifting a radioactive source, exemplified in U.S. Pat. No. 3,500,447, the source is supported on the end of a cable, wire, rod, or wand and is inserted in one direction through a passage to a position directly beneath a sample vial in the counting chamber. In this position the source is received in a radial bore of the sample vial elevator. This approach has not been widely adopted perhaps because of the close dimensions and exacting mechanical tolerances required to coordinate source and elevator movement and to accurately and repeatably insert the source into the elevator recess directly below the sample.